Synchronous wave linear accelerator wherein the slow wave circuit couples only to the positive synchronous wave



Feb. 18, 1969 B w. ZOTTER 3,428,848

SYNCHRONOUS WAVE LINEAR ACCELERATOR WHEREIN THE SLOW WAVE CIRCUITCOUPLES ONLY TO THE POSITIVE SYNCHRONOUS WAVE Filed Sept. 8, 1966 22 &I5 B I FIG,2

INVENTOR, ammo w. ZOTTER 79 ATTORNEYS United States Patent O 3,428,848SYNCHRONOUS WAVE LINEAR ACCELERATOR WHEREIN THE SLOW WAVE CIRCUITCOUPLES ONLY TO THE POSITIVE SYN CHRON OUS WAVE Bruno W. Zotter, Belmar,N.J., assignor to the United States of America as represented by theSecretary of the Army Filed Sept. 8, 1966, Ser. No. 578,440 US. Cl.315-35 Int. Cl. H01j 25/36 This invention relates to linear particleaccelerators and more particularly to a means for accelerating electronsby microwave energy propagating in a transverse mode on the electronbeam.

The acceleration of charged particles in a travelling wave linearaccelerator is usually accomplished by coupling to the fast space chargeor longitudinal wave. One of the limitations of such a system is thatdue to the increase of wavelength with increasing beam velocities, thelength of the drift regions separating the accelerating regions becomesincreasingly longer and the length of each section must be calculatedseparately. Also, the electron beam is bunched in tighter and tighterbunches so that the output from such devices is in the form of bursts ofhigh velocity particles rather than being continuous.

It is an object of the present invention to provide a linear electronaccelerator wherein the above noted limitations are eliminated.

It is another object of the present invention to provide a linearelectron accelerator wherein RF energy is converted into electron energywith relatively high efliciency.

Theoretically, it is known that when a transverse-field slow wavecircuit interacts with an axial electron beam, the beam is found topropagate four waves which are known as transverse electron beam wavesor normal modes. The electron beam is assumed to drift with someconstant velocity ,u in a homogeneous magnetic field B directed alongthe Z axis which coincides with the beam equilibrium position. Two ofthese waves or modes are known as synchronous waves, one of which ispositively polarized. This positive synchronous wave involves onlytransverse displacement and carries positive RF power. The longitudinalvelocity of particles carrying such a positive synchronous wave isgreater than the modulated beam velocity. When the velocity increasebecomes larger compared to the initial beam velocity, the axial orlongitudinal beam velocity will be proportional to the transversedisplacement of the positive synchronous wave from the axis or beamequilibrium position. The present invention is based on this linearrelationship.

In brief, the present invention provides a unique means for acceleratingelectrons by microwave energy propagation in a transverse mode on anelectron beam. It includes a travelling wave tube having means forprojecting an axial electron beam and a transverse-field slow wavecircuit in coupling relationship to the electron beam. Also included aremeans for propagating a microwave signal of relatively high power alongthe transverse slow wave circuit whereby the signal interacts with thebeam to excite a positive synchronous wave on the beam. The slow wavecircuit couples only to the positive synchronous waves whereby theelectron beam energy is continuously accelerated as it interacts withthe coupling slow wave circuit.

For a better understanding of the invention, together with other andfurther objects thereof, reference is bad to the following descriptiontaken in connection with the accompanying drawing in which:

FIG. 1 schematically illustrates a travelling wave particle acceleratorembodying the present invention; and

4 Claims "ice FIG. 2 illustrates a typical transverse-field slow .wavecircuit for use in the travelling wave particle accelerator of FIG. 1.

For purposes of clarity, the required conventional D-C potentials havebeen omitted from FIG. 1 of the drawing. Referring now to FIG. 1 of thedrawing, at 10 there is shown the evacuated envelope of a travellingwave particle accelerator having an electron gun 12 at one end thereoffor emitting an axially electron beam towards a target electrode 14axially spaced therefrom. A longitudinal or axial magnetic field asrepresented by the arrow B is provided in the usual manner by anenergized solenoid 1S and the axial beam is assumed to be initiallydrifting with some constant velocity #0 in the homogeneous magneticfield B. Intermediate electron gun 12 and target electrode 14 is atransverse synchronous wave coupler 16 which is adapted to excite andcouple only to the positive synchronous beam wave or mode hereinabovedescribed. The structural details of a typical transverse synchronouswave coupler 16 known in the art is shown in FIG. 2. As shown, coupler16 is essentially a finned ridged circular wave guide with the fins 18twisted at a prescribed axial pitch which is adjusted to give theoptimum synchronization with the positive synchronous wave. An RF signalof relatively high power is applied to the coupler 16 at the end thereofproximal electron gun 12 by means of a suitable waveguide couplingstructure shown schematically at lead 20. The other end of coupler 16 isterminated by a matched load as shown at 22. The coupler 16 will act asa transverse-field slow wave circuit which interacts with the axiallydirected electron beam so that it excites and couples only to thepositive synchronous wave carried on the beam. Since the positivesynchronous wave carries positive RF power, the kinetic energy of theelectron beam will be increased as it passes through and interacts withthe tranverse-field slow wave circuit provided by coupler 16. Hence, thevelocity of the electron beam is accelerated as it passes through thetransverse-field slow wave coupler 16 so that its velocity as it passesthrough the end thereof is much greater than the initial constant driftvelocity p0 of the electron beam. As hereinabove noted, this increase inbeam velocity is proportional to the transverse displacement of thepositive synchronous wave from the longitudinal axis or equilibriumposition of the beam. In FIG. 1, the displacement of the positivesynchronous wave from the axis as it progresses therealong isrepresented by the broken sinuous line 24. At the terminus of thecoupler 16, the transverse displacement is shown to have thegreatestmagnitude, hence the beam velocity will have maximum acceleration as itleaves the coupler 16. The accelerated beam may be refocussed by asuitable electro-optical lens 30 so that the high velocity beam may bedirected to the target 14 from which X-rays may be emitted due to thehigh velocity electron beam striking the target.

An inherent advantage of the velocity accelerator hereinabove describedis the high efiiciency with which the RF power can be converted intobeam acceleration. Since the coupling impedance of transverse-fieldstructures such as coupler 16 is much higher than that of longitudinalcouplers, acceleration can be achieved over a relatively shorter length.Another advantage resides in the =fact that the beam acceleration iscontinuous.

While there has been described what is at present considered to be thepreferred embodiment of this invention,

it will be obvious to those skilled in the art that various changes andmodifications may be made therein without departing from the invention,and it is therefore aimed in the appended claims to cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What is claimed is:

1. In a travelling wave particle accelerator having means for projectingan axial electron beam within an evacuated enclosure at a constant driftvelocity, means for accelerating the beam velocity, said acceleratingmeans comprising, a transverse-field slow wave circuit in couplingrelationship to said beam, means for propagating a microwave signal ofrelatively high power along said transverse-field slow wave circuitwhereby said signal interacts with said beam to excite a positivesynchronous wave thereon, said slow wave circuit coupling only to saidpositive synchronous wave whereby said electron beam energy iscontinuously accelerated as it couples with said slow Wave circuit.

2. The accelerator in accordance with claim 1 wherein saidtransverse-field slow wave circuit comprises a finned ridged circularwaveguide, the fins being twisted at a prescribed axial pitch.

3. The accelerator in accordance with claim 1 and fur- References CitedUNITED STATES PATENTS 3,239,712 3/1966 Norris 328233 X 3,315,118 4/19627Mi'lller 315-3.6 3,332,024 7/1967 Leboutet 328-233 PAUL L. GENSLER,Primary Examiner.

H. K. SAALBACH, Assistant Examiner.

US. Cl. X.R.

1. IN A TRAVELLING WAVE PARTICLE ACCELERATOR HAVING MEANS FOR PROJECTINGAN AXIAL ELECTRON BEAM WITHIN AN EVACUATED ENCLOSURE AT A CONSTANT DRIFTVELOCITY, MEANS FOR ACCELERATING THE BEAM VELOCITY, SAID ACCELERATINGMEANS COMPRISING, A TRANSVERSE-FIELD SLOW WAVE CIRCUIT IN COUPLINGRELATIONSHIP TO SAID BEAM, MEANS FOR PROGAGATING A MICROWAVE SIGNAL OFRELATIVELY HIGH POWER ALONG SAID TRANSVERSE-FIELD SLOW WAVE CIRCUITWHEREBY SAID SIGNAL INTERACTS WITH SAID BEAM TO EXCITE A POSITIVESYNCHRONOUS WAVE THEREON, SAID SLOW WAVE CIRCUIT COUPLING ONLY TO SAIDPOSITIVE SYNCHRONOUS WAVE WHEREBY SAID ELECTRON BEAM ENERGY ISCONTINUOUSLY ACCELERATED AS IT COUPLES WITH SAID SLOW WAVE CIRCUIT.